The present invention relates generally to gas turbines, particularly to land-based, i.e., industrial gas turbines employing closed-circuit steam or air cooling of hot gas path components and more particularly to a gas turbine having inner and outer turbine shells constructed to provide positive bucket tip clearance control.
In prior U.S. Pat. No. 5,685,693, of common assignee herewith, there is disclosed a land-based, i.e., industrial gas turbine having a turbine outer shell surrounding an inner shell supporting non-rotational parts of certain of the stages. Particularly, the inner shell supports the first and second-stage nozzles, as well as the first and second-stage shrouds. The outer shell directly supports the nozzles and shrouds of additional stages. It will be appreciated that each of the inner and outer shells is formed in circumferentially extending sections about the rotor axis, preferably in two circumferential halves (upper and lower) of 180xc2x0 each. The upper outer shell half and each inner shell half are individually removable from the turbine without removal of the rotor to enable access to the hot gas path components for maintenance and repair. In the above patent, the inner shell is supported by pins extending between the inner and outer shells in a manner preventing circumferential, axial and radial movement of the shells relative to one another while enabling radial expansion and contraction of the inner shell relative to the outer shell for controlling clearance between the shrouds and the bucket tips.
In the above-noted patent, the clearance control system includes a pair of plenums in each of the inner shell halves and which plenums are connected one to the other by a passageway. Particularly, for each inner shell half, the first or forward plenum overlying the first-stage shrouds and bucket tips has an inlet for receiving cooling air, the cooling air flowing circumferentially about the plenum to the mid-line of the inner shell half. Axially extending passages along diametrically opposite mid-lines extend from the forward plenum back to a similar circumferentially extending aft plenum overlying the second-stage shrouds and buckets. An outlet is provided in the aft plenum. Thus, cooling air at steady-state operation from an external air source is supplied to the first-stage plenum inlet for flow about the plenum, axially along the mid-line and about the second-stage plenum to the outlet. It will be appreciated that by flowing a thermal medium in the described thermal circuit, the inner shell may contract and expand in a radial direction in response to flow of the thermal medium. Consequently, by controlling the thermal expansion or contraction in a radial direction of the inner shell relative to the tips of the buckets of the first and second stages, tip clearance control is afforded. With the advent of a further advanced gas turbine design by assignee, there has, however, been demonstrated a need for an enhanced inner shell cooling circuit.
In accordance with a preferred embodiment of the present invention, the advanced gas turbine design includes an inner shell having inner shell halves each having a forward and aft section containing plenums communicating with one another such that a thermal medium may be supplied to one section for flow axially to the other section and return to the one section. Particularly, for each inner shell half, the thermal medium is supplied via an inlet for circumferential flow in a first plenum of the first inner shell half section and for flow generally axially along a first set of passageways in communication with a first plenum of the second section for circumferential flow therein. The first plenum of the second section communicates with a second plenum of the second section whereby the flow reverses direction for flow circumferentially about the second section and then along a second set of axial passageways along the mid-line of the inner shell half to the second plenum of the first section. The flow enters circumferentially extending second passage portions in the second plenum of the first section for exit and return to the thermal medium supply.
Preferably, the thermal medium supply comprises an auxiliary thermal medium source independent of turbine operation whereby the temperature of the thermal medium can be controlled independently of the turbine. By flowing the thermal medium in the described circuit, the inner shell can be expanded at a rate at least equal to or greater than the thermal expansion rate of the rotor and buckets during start-up to prevent contact between the turbine tips and the shrouds and to preclude a rate of contraction of the inner shell less than the rate of contraction of the rotor and buckets to avoid contact between the turbine tips and the shrouds during shutdown. During steady-state operation, the temperature of the thermal medium is controlled to expand or contract the inner shell to minimize the clearance between the shrouds and the bucket tips, affording enhanced turbine efficiency.
In a preferred embodiment according to the present invention, there is provided a method of operating a turbine having a rotor including axially spaced buckets carried thereby forming parts of turbine stages, an outer containment shell, an inner shell about the rotor including nozzles carried thereby forming other parts of the turbine stages, the inner shell including axially spaced sections and shrouds carried by the sections about the respective tips of the buckets of the stages, and a passage formed in the inner shell for flow of a thermal medium to control thermal movement of the inner shell, comprising the steps of forming at least partially circumferentially extending first portions of the passage in the sections of the inner shell substantially at each axial location of the buckets of the respective stages, providing a first passageway in the inner shell connecting the first passage portions to one another, flowing a thermal medium through (i) a first passage portion of one section, (ii) the first passageway and (iii) first passage portion of another section, forming at least partially circumferentially extending second portions of the passage in the sections of the inner shell substantially at each axial location of the buckets of the respective stages, providing a second passageway in the inner shell connecting the second passage portions to one another, connecting the first passage portion of another section with the second passage portion of another section for flowing the thermal medium therebetween and flowing the thermal medium through (i) the second passage portion of another section, (ii) the second passageway and (iii) the second passage portion of one section, thereby controlling the thermal radial expansion and contraction of the inner shell and the clearance between tips of the buckets and shrouds of each stage.
In a further preferred embodiment according to the present invention, there is provided a method of operating a turbine having a rotor including axially spaced buckets carried thereby forming parts of turbine stages, an outer containment shell, an inner shell about the rotor including nozzles carried thereby forming other parts of the turbine stages and shrouds about the respective tips of the buckets of the stages, and a passage formed in the inner shell for flow of a thermal medium to control thermal movement of the inner shell, comprising the steps of flowing a thermal medium serially (i) through passage portions of the inner shell at a first axial location corresponding in part to an axial location of the second stage of said turbine, (ii) forwardly along the inner shell from the first axial location to passage portions of the inner shell at a second location corresponding in part to an axial location of a first stage of the turbine and (iii) rearwardly along the inner shell from the second axial location to passage portions of the inner shell at the first axial locations to control the thermal radial expansion and contraction of the inner shell and the clearance between the tips of the buckets and shrouds of each first and second stages.